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Topic Review (Newest First)

10-26-2012 11:34 PM

Nortonfan

Come to think of it, now, I wouldn't mind having a Sprint to play with. I am reminded, too often, come vehicle registration and insurance renewal time on my herd of vehicles, that I have over-indulged in a shameful manner.

10-26-2012 06:02 PM

cobalt327

The video of the Sprint takes me back! It has a Q-jet- you can just make out the choke pull off and the end of the secondary shaft in the throttle plate, and the shape of the intake runner is much wider on the Q-jet intake. Only 4-barrel engines were called "Sprint". I don't know what the 1-barrel engine was called other than a Pontiac OHC six. The Sprint engines got the chrome air cleaner lid, too.

10-26-2012 09:40 AM

Nortonfan

Quote:

Originally Posted by beertracker

Nortonfan, got any pictures of the Sprint engine? I have always wanted to see what it looked like. bt

I have no pictures of the Sprint L6 engine, but there are many you tube videos that you can watch, and you can get a glimpse of what the engine looks like in the car.

The Original poster might ask some questions there too. But I also have a comment on his needs. I have been monitoring ebay daily for 6 months and haven't seen one listed yet for my 67 ZD Sprint, which is the original OHC H.O version.

There are much cheaper cores available than that particular carb.

Thanks for the referenced link. I had done a fair amount of searching through the various Pontiac forums and could not find much in connection to my project. I did not post or become a forum member to those forums because I thought that this forum (Hot Rod) would have folks with more experience with adaptations for so many different engine configurations; and, so far, it has come through nicely.

I also discovered, from weeks of searching, that there are precious few Firebird Sprint carburetors to be had, and those I did find had a price tag that exceeded the equity in my home. But, it is understandable. On the other hand, the options I have been given by Cobalt347, have put me in a vast field of low hanging fruit. Easy pickings, and in my price range; not to mention the vast collection of knowledge and reference material he generously provided.

Cheers,
Norman C

10-25-2012 10:07 PM

patrickth

Quote:

Originally Posted by beertracker

Nortonfan, got any pictures of the Sprint engine? I have always wanted to see what it looked like. bt

I could post mine, but too lazy to prep it for posting right no. You can see lots of the on the OHC-6 forums.

The Original poster might ask some questions there too. But I also have a comment on his needs. I have been monitoring ebay daily for 6 months and haven't seen one listed yet for my 67 ZD Sprint, which is the original OHC H.O version.

There are much cheaper cores available than that particular carb.

10-25-2012 07:59 PM

beertracker

Nortonfan, got any pictures of the Sprint engine? I have always wanted to see what it looked like. bt

10-25-2012 03:04 PM

Nortonfan

Quote:

Originally Posted by cobalt327

You are quite welcome. One other attribute of the later style Q-jet casting is that the adjustable part throttle (APT) adjustment is easier to access, located under an aluminum plug in the airhorn. The earlier style carb use an adjustment screw in the baseplate and this often is seized up.

More on this here. BTW, try to avoid 1975 and early '76 carbs because they are likely to have the aneroid or spool in them, and this complicates tuning.

Thanks for all this information, Cobalt! Man, I am overwhelmed and gloating! I am holding my search to the model numbers you have provided and find no reason to deviate from your sound advice.

A few searches for suitable carb year/models have proven successful and I see there is plenty of donor carbs/models to choose from. One concern I had (too many to list) was with the larger, dual capacity accelerator pump models. The links (Crankshaft Coalition) you have provided have laid these concerns to rest, however. Thinking, at first, that the plastic connector (blue or gray in color) betrayed the identity of the CCC type carbs; which I have since learned is not necessarily the case. This is quite encouraging, I might add. BTW, am I getting this right, that there is no need to connect anything to this connector atop the float chamber and it will default to the single shot mode with no other ill effects/issues?

The links you have provided have a bounty of information within them. I was up until the wee hours last night doing some 'light' reading. Thanks again for sending them along.

BTW, the transmission on the Mercedes 280C is a 4 speed automatic, but does not require a linkage (cable) to the carburetor. I'm not entirely sure how it all works and I will definitely look into this further... reminder to myself.

Cheers,
Norman C

10-25-2012 02:30 PM

Nortonfan

Quote:

Originally Posted by 75gmck25

I used the Ruggles book and one of his high end rebuild kits (includes more parts) to rebuild a post-76 Quadrajet. The only issue I found was that his book covers so much material on both standard rebuild and performance additions that I was doing a lot of flipping back and forth in the book. I wanted to get a fairly stock carburetor, but also address some performance changes as part of the rebuild.

For example, I removed and tapped the plug for the APT screw so that I could adjust it after I put the carb together. However, it ran well with the stock setting, so I left it that way.

I found the carburetor on an '86 Chevy truck, since they used the non-electronic, non-electric carbs longer on the trucks than on the cars.

Bruce

Thanks for these tips, Bruce. I will keep in mind not to alter the stock configuration of the carburetor until I am absolutely certain that whatever changes I do make are necessary and will be an improvement... though something tells me I will be pulling the carb's cover a few times before I'm finished with the adaptation. Things are looking quite promising, indeed!

Cheers,
Norman C

10-25-2012 07:30 AM

cobalt327

Quote:

Originally Posted by Nortonfan

Thank you, Cobalt327! My goodness, this is splendid information and I can't thank you enough for sending it along.

This may take me some time to digest and be informed enough to be able to comment and ask relevant questions. As previously mentioned, I have ordered a couple of reference books on this subject and the Ruggles book is one of them.

You are correct in saying that the smaller displacement engine would be easier to adjust to the leaner settings, and I must confess that I had my concerns in this regard; not knowing the range of adjustment one could expect and being able to find a suitable jetting/settings from the larger carburetors while still being able to transition smoothly through the other stages of operation. You have provided plenty information here and opened many doors for me.

I will begin my search for a suitable model Q-jet carburetor from the ones you have suggested to see what comes up, and being that the subject models are not likely candidates as "collectible", will make the affordability issue nothing to be too concerned with.

Again, I'm quite stoked and appreciate this very much!

Cheers,
Norman C

You are quite welcome. One other attribute of the later style Q-jet casting is that the adjustable part throttle (APT) adjustment is easier to access, located under an aluminum plug in the airhorn. The earlier style carb use an adjustment screw in the baseplate and this often is seized up.

More on this here. BTW, try to avoid 1975 and early '76 carbs because they are likely to have the aneroid or spool in them, and this complicates tuning.

10-25-2012 03:56 AM

75gmck25

I used the Ruggles book and one of his high end rebuild kits (includes more parts) to rebuild a post-76 Quadrajet. The only issue I found was that his book covers so much material on both standard rebuild and performance additions that I was doing a lot of flipping back and forth in the book. I wanted to get a fairly stock carburetor, but also address some performance changes as part of the rebuild.

For example, I removed and tapped the plug for the APT screw so that I could adjust it after I put the carb together. However, it ran well with the stock setting, so I left it that way.

I found the carburetor on an '86 Chevy truck, since they used the non-electronic, non-electric carbs longer on the trucks than on the cars.

Bruce

10-25-2012 02:18 AM

Nortonfan

Thank you, Cobalt327! My goodness, this is splendid information and I can't thank you enough for sending it along.

This may take me some time to digest and be informed enough to be able to comment and ask relevant questions. As previously mentioned, I have ordered a couple of reference books on this subject and the Ruggles book is one of them.

You are correct in saying that the smaller displacement engine would be easier to adjust to the leaner settings, and I must confess that I had my concerns in this regard; not knowing the range of adjustment one could expect and being able to find a suitable jetting/settings from the larger carburetors while still being able to transition smoothly through the other stages of operation. You have provided plenty information here and opened many doors for me.

I will begin my search for a suitable model Q-jet carburetor from the ones you have suggested to see what comes up, and being that the subject models are not likely candidates as "collectible", will make the affordability issue nothing to be too concerned with.

Again, I'm quite stoked and appreciate this very much!

Cheers,
Norman C

10-24-2012 08:46 PM

cobalt327

As you probably know, there are other things that affect how a carb will work on a particular engine besides the easy-to-change jets and metering rods.

The idle circuit is what mainly needs work, but in your case- having a relatively small displacement engine- this will not be a bother. In fact the idle and transition calibrations used in the later years before EFI (that were often too lean) will actually work good on your engine.

The carb numbers I'm giving you here are for NON computerized/ NON feedback carbs. These were used on light trucks and vans for years after passenger cars all had gone to feedback carbs requiring a computer to run right.

One application that will work good w/o needing a lot of work is a 4.3L Chevy/GMC Q-jet from 1985-86. Here are some specs:

17086046 is same carb internally, only real difference is it has a throttle arm that will allow a TC cable to be attached. I don't know what trans you have so I don't know if you need that throttle arm or not. The difference between an AT and manual throttle arms is the AT arm is extended below the throttle shaft. This allows a cable to be pulled by the arm. But the idle and transition circuits will be good (or very close) as-is, the main changes you may need to make will be primary and secondary metering rods and primary jets along w/the power piston spring. You will adjust the secondary spring and vacuum pull off as needed.

As far as using an original Q-jet for a Sprint engine, they're rather rare and could be expensive. Not to mention they're old and would likely need to have primary throttle shaft bushings installed, plus there's always the chance the castings could be warped, etc.

The newer 1976-up carbs w/the late-type casting are going to be more plentiful and should generally be in better condition. They have improvements over the older carbs like an improved float arrangement and an electric choke. They're also well understood like the older Q-jets and parts and info are readily available.

Before going further, are you actually working with a Pontiac Sprint or Sprint clone, or are you tuning a different but similar set up?

Sorry I forgot to mention this earlier. Actually, the application is for a '75 Mercedes 280C, DOCH with in-line 6, high revving (6500 RPM), 2.8L engine. The carburetor (Std.) used by Mercedes for this application was a spreadbore, Solex 4A1 which is a poor and disappointing copy of a Quadrajet in every respect and leaves much to be desired. I will need to replace this carburetor as it is beyond repair (severely warped) and leaks fuel and air/vacuum making it un-tuneable (is that a word?), and miserable to drive. In any case, I would prefer to invest my time, money and energy into finding a permanent cure to the Solex issues; thus my inclination into staying focused on converting/modifying a Q-jet to suit this purpose.

In terms of air flowing capabilities, I have no idea what the Solex's CFM raitngs are; though I have searched in vain for this information. However, from what I can tell, the venturis and bores look to be substantially equal if not identical, as well as the exterior appearance. I have managed to gleen from extensive searches, that the carburetors that comes very close to matching the Solex, in terms of air delivery are the ones found to have been fitted to the Pontiac Firebird Sprint. Thus, my search for additional information on these carburetors begins here. Those who have managed to successfully make the Q-jet adaptation to the Mercedes 280C appear to be quite happy with the results, but are no longer available to offer/share the finer details.

I have Webers 32/36 and 38/38, Autolite 4100 and Holley 4175 carbs available to me in my stash of good-to-haves. The only one of these I would spend time attempting to convert would be the Autolite 4100 (1.12 venturis); failing my ability to make the Q-jet work. That said, it may be my last resort but only because of the availability, simplicity and low cost of these old 'shoe boxes'. Converting to a Q-jet is quite a daunting challenge, no doubt. But isn't that what hot-rodding is all about?

I have recently ordered a couple of books on the subject of tuning and servicing Rochester Q-jets, but nothing like being able to bounce ideas off of kindred spirits in one's pursuit of knowledge. I did not mention that I have a well equipped home workshop with machining and welding capabilities; if nothing else, it may come in handy to machine a set of bushings for those pesky worn out throttle shaft bores.

Thanks for the info you have provided, I very much appreciate it. I look forward to any additional thoughts and information you may have and be willing to share.

Cheers,
Norman C

10-24-2012 03:34 PM

cobalt327

Hi Norman. I had a '68 LeMans w/the 215 hp Sprint engine. Actually ran pretty good for only 250 cid.

Before going further, are you actually working with a Pontiac Sprint or Sprint clone, or are you tuning a different but similar set up?

10-24-2012 12:07 PM

Nortonfan

Quadrajet tuning '67-'69 Firebird Sprint

Hello to all,

My first post here and hope I don't get anyone upset with my questions.

I would like to 'convert' or tune a quadrajet carburetor (yet to be acquired) to match the specifications of the ones that orginally came on the six cylinder models of the Pontiac Firebird Sprint, say, between '67 - '69. I have searched unsuccessfully for information on jetting, metering rods, etc, though I have found the applicable model numbers for these carbs., which are practically unobtainable and, if available, way beyond my stash of cash on hand for this project. And, yes, I have considered Holley, Autolite, Weber, Carter and Edelbrock. I have no prejudice or inclination for or against any of these; however, I do favor the Quadrajet on account of its inherent tuneability and proven performance, gas economy and reliability, once properly set up. Not to mention cheap!!! Which is where my finances allow me to hang out. Not a sob story, just dealing with facts here.

I realize there is more to tuning these carbs beyond float levels, jet and needle replacements ... and lots of patience! But, I have considered the alternatives and would only opt to switch if completely discouraged in finding the information I need.

The carb part/model numbers I have found pertinent to the Pontiacs in question are: 7027268, 7027260, 7028260, 7028261, 7029260 and 7029261. I believe these are all 750 CFM. I understand that some of the earlyer versions of these carbs for this specific application, were equipped from the manufacturer with parts that are no longer available. The numbers listed, however, are of the later versions.